The three angles of a triangle are in the ratio 1 : 2 : 3. The sides opposite them are in the ratio:

The angles are 30°, 60° and 90°. By the sine rule the sides are proportional to the sines of their opposite angles: sin30° : sin60° : sin90° = 1/2 : √3/2 : 1 = 1 : √3 : 2. Sides grow with the sine of the angle, not with the angle itself, so 1 : 2 : 3 is the classic trap.